The 3rd-Generation Partnership Project (3GPP) is standardizing the Long Term Evolution (LTE) Advanced radio access technology. Cells are identified in an LTE-Advanced system, at least in part, using a Physical Cell Identity (PCI) that is not globally unique in the system. In fact, only 504 different Pas exist in the system. Limiting the number of Pas simplifies the cell searching process of user equipments (UEs), but inevitably leads to reuse of the same Pas in different cells.
One complication that results from PCI reuse is termed PCI confusion. PCI confusion arises when a UE in a serving cell reports neighbor cell signal measurements to the serving base station (called the serving evolved NodeB, eNB, in LTE terminology). The UE associates reported signal measurements with the neighbor cells on which they have been performed by including the Pas of those neighbor cells in the report. If two of the neighbor cells have the same PCI, the report's association between signal measurements and neighbor cells is ambiguous. This proves particularly problematic in the case where the serving eNB uses those measurements for handover of the UE to one of the neighbor cells, because the serving eNB may inadvertently hand over the UE to the wrong neighbor cell and thereby cause radio link failure.
In an effort to avoid these problems, cells in the LTE-Advanced system are also identified by a globally unique identifier referred to as a Cell Global Identity (CGI). When PCI confusion occurs in a UE's measurement report, the serving eNB can instruct the UE to acquire the CGI of the problematic neighbor cell from system information broadcast by the cell. This CGI will resolve the PCI confusion at the serving eNB. However, the UE may have to briefly interrupt its transmissions in order to acquire the CGI.
eNBs in an LTE-Advanced system thus store neighbor cell information within so-called neighbor relation tables (NRTs). Each NRT includes, among other things, a mapping between a neighbor cell's PCI and CGI. If an eNB receives a measurement report that causes PCI confusion with respect to a neighbor cell for which the eNB has a stored NRT, the eNB can resolve the PCI confusion by referencing the NRT rather than by having the UE acquire the neighbor cell's CGI.
An eNB must also reference an NRT for certain types of signaling in an LTE-Advanced system. Signaling between eNBs via a Mobility Management Entity (MME), for example, requires identifying cells with their CGIs because the MME routes messages based on part of those CGIs. Further, handover messages sent over an X2 interface between eNBs (called X2 handover request messages), and handover messages sent over an S1 interface between an eNB and a MME (called S1 handover required messages), also require identifying cells with their CGIs. Still further, an eNB can use a target cell's CGI to recover the target eNB's IP address and establish an X2 interface to that target eNB.
A serving eNB automatically populates its NRTs by requesting reports from served UEs that are located at different spots near the coverage limits of the serving cell and that thereby border different neighboring cells. Upon such request, the served UEs decode and report the CGIs of the different neighboring cells. This so-called UE Automatic Neighbor Relations (ANR) process takes considerable time in order to acquire the CGIs of all neighboring cells.
The fact that the UE ANR process takes time introduces complexities to systems that utilize relay nodes (RNs). From a radio propagation perspective, a relay node (RN) is positioned between a donor eNB and one or more UEs. An RN connects to the donor eNB using the same, standard radio link used by ordinary UEs. The RN then provides radio access to UEs, effectively emulating an eNB from the perspective of the UEs, and uses its radio link to the donor eNB as backhaul transport for UE data. As part of this emulation process, a cell provided by an RN (i.e., an RN cell) appears to a UE as a separate cell that is distinct from the cell provided by the donor eNB (i.e., the donor cell). An RN cell, for example, has its own PCI. An RN may also maintain and populate its own NRTs in a manner similar to that for eNBs.
Heretofore, RNs have remained statically fixed in a particular location, just like eNBs, so the delay involved in populating the NRTs using UE ANR has not introduced new challenges. However, new challenges are in fact introduced by recent proposals to mobilize RNs so that they effectively roam about the system like UEs.
Specifically, mobile RNs are to be installed on or inside of trains, buses, and other moving vehicles. UEs travelling in such a vehicle connect to the mobile RN rather than an eNB. Accordingly, instead of having to handover from eNB to eNB as the vehicle moves, each UE can remain connected to the mobile RN. The only eNB-to-eNB handover that need take place is of the mobile RN, which can be accomplished in a manner similar to that of UEs. Handover of just the mobile RN, rather than multiple individual UEs, optimizes radio signaling and reduces handover failures.
However, the transitory nature of a mobile RN inherently limits the amount of time that the RN has to populate its NRTs using UE ANR, since the mobile RN's neighbor cells vary as the RN moves. If the mobile RN does not have enough time to complete its NRTs, a likely scenario in many use cases, the RN will have to request that a UE read a neighboring cell's CGI in order to resolve PCI confusion from a UE's measurement report. In addition to the disadvantages mentioned above, that in turn delays any handover of the UE that may be made based on the report, such as a handover from the mobile RN to an eNB once the UE's user disembarks from the vehicle to which the mobile RN is attached. Such delay is particularly unacceptable in this context because the radio link between the UE and the mobile RN may deteriorate quickly once the vehicle moves away from the UE.